A common type of switched mode power supply (SMPS), for example, a flyback SMPS, includes a transformer, a semiconductor switch, and a control circuit. The transformer provides a voltage conversion from the available input voltage to the desired output voltage. The semiconductor switch, which may suitably be a power MOSFET, switches at a high frequency to create a high frequency signal that can efficiently propagate through a transformer with limited loss. The duty cycle and/or switching frequency is controlled by the control circuit to accommodate different loading requirements on the output.
Commercially available integrated circuits exist that may include the semiconductor switch and the control circuit in a single package, along with other features of an SMPS. One set of packages include the switch mode power supply ICs from ON Semiconductor, such as the NCP101x family, the NCP105x family. These devices include a switch input to which the primary winding of the transformer is connected, a switch, a bias input for providing bias power, and a controllable current source coupled to provide current to the bias input to allow configurations for the IC to be powered off of the line current. In other versions of similar devices, a switched mode power supply IC can include the control circuit, drive circuit and current source, but employs an external switch.
One design consideration in switched mode power supplies is the electromagnetic (EM) emissions created by the high frequency switching signal. One method of meeting conducted emission requirements involves jittering or varying the switching frequency. In general, jittering the switching frequency operates to spread the EM energy over a range of different frequencies, such that the power is not concentrated in a single frequency or a single set of harmonics.
However, in some commercially available SMPS circuits, such as those discussed above, the varying of the switching frequency is only available when the circuit is powered using AC line voltage. The problem with powering the IC from the line voltage is that when the line voltage is high, such as in many metering applications, a large quantity of power is dissipated in the SMPS circuit just to make it run. The dissipated power significantly limits the amount of power it can provide to the load without exceeding its recommended operating temperature.
To avoid the temperature limitations, methods other than varying or jittering the switching frequency can be used. Many such solutions involve powering the bias input (and the switch IC) using an external source of power, such as one based on an auxiliary (lower voltage) winding of the transformer. In such a case, instead of jittering the offline SMPS switching frequency, solving the EM problem is carried out using a common mode filter across the line and neutral inputs of the power supply to reduce the average conducted emissions. The auxiliary winding provides the current needed to power the SMPS, resulting in very low power dissipation.
A second way also involves using a low voltage auxiliary winding coupled to the bias input. However, in this way, the EM emissions are reduced by using a dedicated timing circuit inside the IC to jitter the switching frequency instead of relying on the sawtooth waveform on the VCC pin. The inclusion of a dedicated timing circuit increases cost.
There is a need, therefore, for an efficient way of spreading the EM emission energy over a range of frequencies that has reduced power consumption and does not rely on special additional timing circuits within an integrated circuit.